CERTAIN QUANTITATIVE RELATIONS OF NITRATE AND NITRITE METABOLISM IN FARM ANIMALS WITH SPECIAL REGARD TO THE ORIGIN AND DEVELOPMENT OF METHAEMOGLOBINEMIA CAUSED BY NITRITES AND THE DIAGNOSIS OF POISONING : II. NITRITE REACTION WITH HAEMOGLOBIN IN THE BLOOD OF FARM ANIMALS IN VITRO AND IN VIVO. REDUCTION RATE OF METHAEMOGLOBIN IN BLOOD